Grinding wheel



Dec. 14, 1937. K. F WHITCOMB AL GRINDING WHEEL Filed Jul :5, 1935 5 m." E H MW T m N m N MG N n N m m d m 8 w E6R a E T T 6 N n E L C A G M U 0H F m Fm T NZMW E C N m M D 4 M2 mpmim Emmi KENNETH F. I/VHITCDMB Z HERBERT W. WAGNER Patented Dec. 14, 1937 UNiTED STATES GRINDING WHEEL Kenneth F. Whitcomb and Herbert W. Wagner,

Worcester, Mass, assignors to Norton Comnany, Worcester, Massachusetts Mass a corporation of Application July 3, 1933, Serial No. 678,816

13 Claims.

his invention relates to a grinding wheel, and particularly to that type of wheel which is made of abrasive grains united into an integral body by a suitable bond, such as rubber, shellac, the resinoids, sodium silicate and vitrified ceramic materials.

The primary object of this invention is to provide a grinding wheel of standard grinding characteristics and of uniform structure throughout its effective grinding zone, but which is so reinforced that it may be rotated at a rapid rate or used under severe temperature conditions without breaking, and which will serve with high efliciency in a grinding operation. Other objects will be apparent in the following disclosure.

The two major destructive forces which tend to break a grinding wheel are the centrifugal force due to rotation of the wheel and the stress set up by differential temperatures in the wheel resulting from the heat of grinding on its periphery. These influences build up their most destructive efiects adjacent to and around the central hole of the wheel. This invention, therefore, contemplates fortifying the wheel against breakage by making the inner zone or central portion of the wheel around the hole more resistant to these forces than is the grinding zone of the wheel, whose properties are fixed by the grinding characteristics required for a given operation. Such a grinding wheel may comprise at least two zones, an outer annulus which serves for the grinding operation, and an inner zone adjacent to the arbor hole whichis more resistant to the sum of the two stresses than is the material of the, grinding zone, or to that stress which is predominant and tends to cause breakage of the wheel.

In the drawing:

Fig. 1 is a set of curves illustrating certain stresses in a grinding wheel shown in Fig. 2;

Fig. 2 is a section through a composite twozone wheel, arranged to correspond with the curves of Fig. 1;

Fig. 3 is a plan view of the wheel;

Fig. 4 is a detail of the joint between the two zones; and r Fig. 5 illustrates a mold for shaping a two-zone wheel.

In this multiple zone wheel, the outer grinding zone must be made in accordance with the desired grinding characteristics, as regards thenature and kind of the abrasive, the grain size, the nature, type, and quantity of the bond, the volume percentages of the abrasive, bond and pores, the wheel dimensions and uniformity of thickness and other fixed characteristics 'of the wheel. Hence, this outer zone structure cannot be changed. It is, therefore, desirable that the characteristics of the inner strengthening zone of the grinding wheel beso selected as to meet 5 the various requirements determined by the conditions under which the wheel is to be manufactured and used, so as to prevent cracking or a strained condition at the joint and to strengthen and form an integral body with the outer zone. In the manufacture of a vitrified grinding wheel, the requirements of the average manufacturing operation indicate the desirability of having the same bond and type of grain in the twozones, so that the wheel may'be easily manufactured as a unit; but if desired, and within the scope of this invention, the two zones may be made of radically different materials, provided all or the more important of the fundamental conditions outlined below are satisfied. For example, a vitrified wheel may be provided with a rubber or a synthetic resinoid center, with or without abrasive material therein, as hereinafter explained. Likewise, other wheels may be strengthened by means of centers which are adapted for the purpose. The invention will, however, be described specifically with reference to the manufacture of a ceramic bonded wheel in which the bond in each zone is a ceramic material capable of being vitrified to a glassy or porcelainie condition in the presence of the abrasive grains.

Strength of wheel structure The strength of the wheel structure depends upon the grain size and the quantity and type of bond and grain employed. As the grain size decreases, the strength increases within certain limits, if the volume structure of the wheel remains the same. However, after the grain size has been decreased to a certain point, .the maximum strength per unit of volume. is obtained. Experimental work has developed the fact that this maximum strength of a ceramic bonded article is approached when the grainparticles have been reduced to approximately that size at which the grains. will just pass through a screen of 100 meshes to the linear inch or somewhat finer. Therefore, the inner zone may be strengthened by using a finer grain size than found in the 50 outer zone, and preferably as fine as 100 mesh. Coarse grains may be interspersed with the fine grains, but a considerable amount of the grains should be fine. It is also possible to increase the strength of the inner zone by the use of more 55' '.center of the wheel.

bond or a bond of greater intrinsic strength, provided it will unite integrally with the material of the outer zone. The wheel structure may be strengthened,. therefore, by:

( 1) Employing'a finer size of grain in the central zone than is used in the grinding zone, and

preferably while maintaining the same volume percentage of abrasive in the two zones.

(2) Using the same grain size and the same volume percentage of bond in the two zones, but having a greater amount or volume percentage of grain in the inner than in the grinding zone.

(3) Having a higher bond volume percentage in the inner than in the outer zone.

(4) Using a bond in the inner zone which has' a higher intrinsic strength than that used in the outer zone. I

(5) Employing a combination of the above features.

Relative sizes of the two zones A further factor which should be considered is the ratio of the diameter of the reinforcing or inner zone to that of the outer grindingzone. It is desirable that the reinforcing inner zone be as large'as practicable. The ideal wheel is one in which the material of the inner zone is so selected-that, as the stress in the outer zone at the edge of the-joint approaches the ultimate strength of the outer zone material, the inner zone is of such strength and size that, although it has strengthened the-wheel, it will break just before the outer zone breaks, and this break will start at the edge of the hole.

' Fig. lshows atypical stress distribution in a standard grinding wheel of a given structure, 11-

lustrated in Fig. 2 in correct relationship to the curves, which has a radius of 12" and a small hole of 1" radius. As there plotted, it will be apparent that the tangential stress due to rotating the wheel at a high speed and the stress set up by the frictional heat of grinding are very high"- at the edge of the hole l0, indicated by the line a in Fig. 1, and that the major portion of this stress is found within a short distance of the It will, therefore, be appreciated that the inner zone of the wheel need not extend radially outwardly beyond that point at which the stresses have become comparatively small: In such a wheel, an'inner zone having a radius equal to one-third of the radius of thewheel body, 1. e., the distance from the edge of the hole-to the periphery of the wheel, will be subjected to the major portion of the disruptive forces met in a grinding operation,'as indicated by the line b in Fig. 1, which corresponds with the position of the joint -Il between the outer zone I2 and the inner zone l3 of the wheel shown in Figs. 2 and 3. This inner zone may be made larger or smaller, proportionate to the outer zone, as determined by. the particular characteristics of the grinding wheel. under consideration, but ordinarily it should not make up more than half of the radial thickness of the wheel. In particular, the inner zone should not extend into that space normally assigned for grinding use, and it is preferable that it lie wholly within the outer edges of the flanges or plates which support the grinding wheel on the spindle.

Requirements at the joint between the zones To make this invention practical, certain requirements at the joint or circular junction of the two zones must be taken into account in order that separation of the bodies of the two zones changed.

may beprevented, as well as creation of a high jacent portion of the other zone. To accomplish this; matching of thevolume changes, the bodies of the two zones in an ideal wheel should have approximately equal values for each of the followi'ng'properties, as explained below:

(1) Shrinkage due to drying and maturing I .i

the bond during manufacture of the wheel (2) Coeflicient of thermal expansion of the body during manufacture and use (3) Modulus of elasticity during manufacture and use of the wheel.

If the properties (1) and (2) are not near enough equal. one zone. will be subjected to a stress tending to produce a crack between the two zones. deformation of body set up at the joint, by centrifugal force ordifierential heating, will deform the materials of both zones at the joint. If the -modulus of elasticity of the innerzone is much 'higher' than that of the outer zone, the outer zone material, being less stifl, will accommodate more than its share of the required deformation, and tend to reach its breaking pointtoo soon.

which would result in premature failure of the.

.wheel.

Shrinkage during drying and maturing the bond As above. stated, it is desirable that the drying and firing shrinkages of the two portions be equal or so matched that the parts will remain in proper relation to one another, during use as well as manufacture of the wheel. Some of the factors to be considered are:

(1) The volume percentages of abrasive and of bond in both zones while in the green un- (2) The kind of abrasive and its size and shape and the kind of bond in each zone;,

(3) The amount of removable .constituents, such as water or other plasticizer required to make a moldable mixture of the grains and bond; and

(4) The variations in heat treatments and maturing temperatures of the bond, so that two zonesmay be formed at the same time.

An ideal composite wheel is one where the drying and firing shrinkages of the outer and inner zones are the same. Some differential shrinkage is permissible 'if the strengths of the two materials are 'suiiicient'to prevent rupture.

although a strained condition is set up. It, however, is necessary to select the proper materials for the inner zone so that its characteristics will match those of the outer zone which cannot be Coeflicients of expansion In the manufacture of a vitrified wheel, in which the vitriflable ceramic material is fired to a fluid condition and then solidified, it is necessary that the two zones of material contract at substantially the same rate during the In regard to the third property, a-

heating up of the wheel during its storage or use.

The coeflicients of expansion ofthe bond and the individual grains should preferably satisfy the conditions set forth in the Patent to Saunders, Milligan and Beecher No. 1,829,761, dated Novemher 3, 1931. The present considerations, however, apply to the substance of each zone taken as a whole and refer particularly to the joint along which the materials of the two zones lie in contact. It is necessary that the inner zone shall not contract more rapidly than the outer zone and thus pull away from it'. On the other hand, the outer zone should not shrink more than does the inner zone and to such an extent as to set up strains within the outer zone which will tend to rupture it, during either the cooling operation or the subsequent use of the wheel.

It will, therefore, be appreciated that while the ideal conditions arenot always practical, it

Modulus of elasticity The modulus of elasticity may be defined as the ratio of the stress applied at rupture of the wheel.

to the percentage of elongation produced by that force, and it is a measure of the stiffness or resistance' to distortion of the material.

Theideal composite grinding wheel is one in which the structures of the two zones are such that their moduliof elasticity are substantially equal, or as nearly equal as the other. factors permit. However, it is found that the modulus of elasticity of either zone of a vitrified wheel may be as much as 2.6 times as great as that of the other zone; but it is preferable that the modulus of elasticity be the greater in the outer zone if equality is not feasible. In such a case, the outer zone would have the greater stiffness, and the inner zone would have the greater ability to stretch under the forces applied to the wheel by rotation and heat applied at the periphery. This theoreticalvalue, which is subject to experimental error, may be departed from widely because of other considerations ,which may control the manufacture of the wheel; and a, reasonable latitude of interpretation of the claims is, therefore, to be had.

Ability of the inner zone to stretch be greater than that of the other zone, in order to resist heat breakage. In other words, where the two zones have the same moduli of elasticity, the-inner zone will have the greater strength and the greater deformation at rupture, as indicated by Equation (1) That is, if the inner zone has twice the strength but the same modulus of elasticity as has the outer zone, then it can stretch twice as much at the edge of the hole at rupture as would a wheel made wholly of the outer zone material. Since the moduli are the same, the material on eachside of the joint will stretch the and the center.

will be readily understood.

same, although one is stronger than the other, and the wheel cannot pull apart laterally at the joint. If two dlflerent grinding wheels have the same tensile strengths, but different values for deformation at rupture, they will have unequal resistance to-heat breakage; and that wheel which has the. greater ability to stretch will withstand the higher temperature diflerence. For example,

an organic bonded grinding wheel, such as one made of crystalline alumina grains bonded by means of a synthetic. resinoid (e. g., a heat-set phenolic aldehyde condensation product), may have the same tensile strength as a grinding wheel made up of the same abrasive grains bonded by a vitrified ceramic material; but because of the fact that the resinoid has the higher value for its deformation at rupture, it will stand greater temperature differences between the periphery n the other hand, it will be appreciated that a mere ability to stretch is insufiicient to withstand centrifugal stresses, since an inner zone of soft rubber could not strengthen the wheel. The material must have both stiffness and strength to resist breakage in use.

Compositions and structures of composite wheels In accordance with this invention, it is proposed to make composite grinding wheels which satisfy the .major conditions above set forth, and preferably all of them. To manufacture such wheels successfully and on a commercial basis, it is necessary that the structures of the two or more zones in the composite wheel shall haye' such physical properties and bear 'such relationship to each other as to avoid a. strained condition at the joints between the various zones of the wheel; and the, wheel structures should'be so constituted that these properties remain constant throughout the life of the wheel. It is, therefore, desirable in accordance with this invention to employ what is known as the controlled -structure method of manufacturing such a wheel, as fully set forth in the patent to Wallace L. Howe and Richard H. Martin, No. 1,983,082, dated December 4, 1934, and the British Patent No. 385,328. In accordance with that procedure, the volume percentages of the granular material, the bond and the pores of the final product are accurately precalculated to give the desired wheel characteristics.

cedure comprises determining'the volume which the granular material and the bond will occupy in the final product in each of the zones, taking into account the shrinkages involved in molding and shaping and drying the wheel and later in maturing the bond, such as is involved in firing a mixture of ceramic materials to vitrify the same.

This prov The next step, based on these prec'alculations, is

to determine that volume which the raw materials in intimate mixture shall occupy in the mold while in a green condition, so that when the bond has been matured,'the body of each zone will shrink to the precalculated volume. The operation of causing this molded mass of green mate- .rial to occupy the required volume in the raw con- Ceramic materials of any desired formulas, or which are of standard compositions, may be used in accordance with these calculations. In this inner zone, one may use either the same abrasive material as employed in the outer zone or, if desired, any suitable granular materiaL'and preferably substances which are not expensive but will satisfy the conditions set forth above. It is feasible to employ either the same general type of bond as used in the outer zone, or one may use altogether different bonds. [For example, if the outer zone bond is a vitrifiableceramic material, the inner zone maybe made of a similar or .diflferent ceramic materials, or it may be made of rubber, the artificial resinoids or other materials, with or without abrasive grains or fillers, and which are standard bonds in this art and need not be fully described herein. A

vitriflable ceramic material may also be used.

alone without any granular material for the inner zone, provided the characteristics of the porcelain or glass satisfy the above conditions.

Various expedients may be employed which will aid in uniting the materials of-the two zones.

-' It, for example, is feasible to employ a lapped joint, as illustrated in Fig; 4, in which case ,the joint between the two zones is staggered so that teeth, as it were, on each of the zones will interlock with and project into the material of the other zone and contact intimately therewith.

As a simple illustration of a structure which is suitable i'orv the purpose of molding these two zones, reference is to be had to Fig. 5 of the draw- 1113. As thereshown, a mold cylinder 20 of suitable internal dimensions may be'placed on a mold plate 2|, and a cylinder 22 of required dimentions to provide the inner zone isplaced therein. This inner cylinder 22 is short, as illustrated, and is spaced from the mold plate 2|. This may be accomplished by providing the inner cylinder with four radial strips 24 of equal length, which are spaced around the outside of this cylinder 22 and are integrally joined thereto. These serve to hold the inner mold in its correct position. These four equally spaced small strips are set upon wooden blocks 25 of approximately l" thickness.

Having now arranged the parts in this fashion, the mixture of material 26 for the inner zone is then poured into the 'inner' cylinder 22 to a depth of about, 1", and it will flow under the edges of the cylinder until it reaches the angle of repose. Then, the outer zone material is poured into the annular space around the inner ring. to a depth of about 1 thus covering the inner zone material that has spread out under the inner mold wall. The inner cylinder is then moved upwardly for a short distance, and the operation repeated. These steps are done sue, oessively until the outer mold has been filled with the material, as required. This method results in the zigzag joint shown in Fig. '4.

Following this operation, a cover plate is placed within the'mold 20 and subjected to pressure to compact the material and squeeze the pores able kiln and heated to a vitrifying temperature to such an extent that the two'zones' are compelled to occupy that total volume which has been precalculated for the wheel of this particular structure. Thereafter, the molded material is removed from or left in the mold, depending upon the final bond maturing operation, and the bond is matured. In the case of a ceramic bonded wheel, the wheel is removed from the mold and properly dried, and then placed in a suitas required for the particular bond being used which, for the standard vitriflable bonds, is ordinarily from Seger cones 4 to 13. In the case of a resinoid or rubber bond, the material may be left in the mold and there heat treated under pressure or as desired'to heat-set the bond. Such bonds may be matured in accordance with the standard methods.

Wheel compositions It will be apparent, in view of the above statements that grinding wheels will differ materially in their compositions and structures and that consequently. the composition of the inner zone of eachwheel will'necessarily be determined by the composition and structure of the outer zone. As an example of one particular composition and structure, we may assume that a particular grinding operation requires a wheel of 24" diameter, 2" thickness and 2" hole. The grain size in the grinding zone may be such as will just pass through an 18 mesh screen and be designated 20 mesh. This wheel may be made'of either silicon carbide or crystalline alumina abrasives and bonded by suitable materials. In the present instance, :crystalline alumina abrasive and a ceramic bond are chosen. A suitable bond may have the following composition:

To obtain a desired grade of hardness, 2% ounces of the green ceramic bond mixture'may be used per poundof crystalline alumina abrasive, together with sumcient water to render the mass -moldable. The amount of abrasive and bond are so proportioned that the resultant grinding wheel will have 54% by volume of abrasive in the outer grinding zone. When this mix ture of bond and grains has been vitrified in the ceramic kiln at a: suitable firing temperature, the vitrified structure of the outer zone has approximately the following properties:

'Modulus of elasticity=9.4 10 lbs. per sq. in.

Tensile strength==1900 lbs. per sq. in.

Shrinkage (volumetric) =0.9%.

but has a. greater strength, thus giving a greater deformation at rupture in the inner zone and thereby strengthening the wheel. The modulus of elasticity of the inner zone may be varied to be smaller than that of the outer portion, but

65 Coefllcient of expansion=0.0000067 per deg reef Coemcient of expansion for the sake of making the two zones unite integrally without strain, it is desirable that they have the same coefficient of expansion and shrinkage. To this end, the inner zone mixture ,may be made of abrasive grain of 36 grit size;

the structure may be one which has 52% by volume of abrasive; and 3% ounces of the same bond as used in the outer zone may be employed per pound of abrasive. This composition, when fired, will have a tensile strength of 2900 lbs. per sq. in., as distinguished from the 1900 in the outer zone, thus producing a far stronger center zone than is the outer zone. If it is desired that the central zone be 12" in diameter or extend half way radially outwardly, the proper amounts of the mixtures by volume and weight are proportioned as follows:

Outer zone Inner zone 24:21:12 12x2x2" 699cu.in. 200.8 cu. in. Weight of raw mixture. 64.3 lbs. 18.5 lbs.

previously fashioned vitrified wheel and then be set to a hard condition by a combination of heat and hydraulic pressure, as is well known in the art. The center of the vitrified wheel may be initially painted with a suitable solution of the resinoid so as to promote penetration of the inner zone material into the pores of the vitrified body and its adhesion thereto. The resinoid may include other fillers, such as quartz, which serve to strengthen it or otherwise improve its properties, such as by cutting down the shrinkage of the material when it is hardened. Abrasive material, and particularly the fine sizes, may be used with the resinoid in accordance with the principles outlined above.

Similarly, a grinding wheel, such as a vitrified wheel as above described, may be strengthened by a hard rubber center. As an example, a grinding wheel may be made according to standard procedure of abrasive grains bonded by a vitrifled ceramic material, but with the hole of a suitable oversize. Within this hole, a strip of rubber compound capable of vulcanizing to a hard condition is placed in contact with the peripheral surface of the wheel hole-and then a suitable rubber compound, such as a mixture of one part by weight of rubber to seven of abrasive grains, is placed between the strip and an arbor of. desired diameter. The abrasive may be crystalline alumina or other material, and preferably of a fine size, such as 60 grit or finer. The rubber compounds are made in accordance with standard procedure, and they are used in such amounts that when pressed, they will fill the space between the arbor and the surface of the Outer grinding zone Crystalline alumina abrasive Vitrifiable bond of the composition given in the above table Structure Modulus of elasticity Tensile strength"--- Shrinka e Coefiicient' of expansion -5 2 of so and or'se grit size I 3% ounces of bond per pound of crystalline Inner grinding zone Crystalline alumina abrasive Vitrifiable bond, same as in the outer zone Modulus of elasticity Tensile streng h 7 Shrinkage of 60, A, of '10 and M; of 80 grit size 3 ozs. bond/lb. of abrasive .7 x 10 lbs/sq. in.

3800 lbs/sq. in.

Proportions in completed wheel Outer zone Inner zone Zone size 30 x 3 x 15 x 3 x 3" Volumes. 1564. 5 cu. in 511. 2 cu. in. Weight of mixture. 141. 6 lbs. 45. 0 lbs.

A grinding wheel may have a center of other 0.0000067/ deg. C.

sired within the capacity of a rubber compound.

of the type used.

As a further example illustrating the breadth of this invention, a resinoid bonded wheel of the softer grades may be strengthened by means of a core of a \harder resinoid material. In the outer zone, one may have, for example, a mixture of 86.9% by weight of abrasive of 12 grit size and 13.1% of "Bake1ite,resinoid pressed to a volume having 2.56 grams per cubic centimeter. The inner zone in such a wheel may be made of the same material but of a finer grit, such as #60. The inner zone will be so proportioned and compressed as to have the same volume structure as is found in the outer zone. In this case, each will have 56% by volume of abrasive, 26% of bond and 18% of pores. For a much softer wheel, one may use the same composition for the center zone, with an outer zone having 90.2% by weight of abrasive and 9.8% of bond. One skilled in the art will know how to vary the proportions of the ingredients and to choose such methods of manufacture as will make other types of wheels as required in the industry. This invention is not limited to any particular type of wheel or method of manufacture, and various other modifications of this invention will be readily apparent in view of the above disclosure.

It will, therefore, be appreciated in view of the above disclosure that we have provided a grinding wheel comprising an outer annular zone of bonded abrasive material of required grinding characteristics and an inner zone integral therewith which may be made of various materials, provided they satisfy certain characteristics. These characteristics are that the inner zone have such strength, ruptural deformations and modulus of elasticity as compared with the respective characteristics of the outer zone that the composite structure will be rendered capable of withstanding a higher breaking speed or possessing greater resistance to breakage due to frictional heat at the wheel periphenf, or both, than will a wheel of the same size made of the outer zone material. It is preferable, as above explained, that the inner zone have the higher strength and ruptural deformation and that the moduli of elasticity of the two zones lie within the ratio of 1 to 2.6, but it will be appreciated that each one of these three primary characteristics.

may be suitably modified if the net result is one of strengthening thewheel. Also, the other characteristics discussed above will be suitably controlled within the requirements of the particular wheel being manufactured. Hence, the invention claimed herein is to be interpreted as limited to the various methods, compositions and structures, as well asthe above defined characteristics, which are disclosed above for strengthening a grinding wheel, together with such modifications or equivalents as will be apparent to one skilled in the art.

The bonds in the two zones are deemed to be of the same class, as specified in the claims, when the bonds are both rubber compmmds, or

' both resinoids, or both vitrified ceramic materials, etc., without regard to variations in the specific proportions or compositions of the two bonds. For example, if vitrifiable ceramic mixtures are used to make the bonds of the two zones, the composition in the inner zone may be materially different from that used in the outer zone, as is required to give the desired streng h and other characteristics.

Having thus described the invention, what is claimed as new and desired to obtain by Letters Patent is: v

l. A composite grinding wheel comprising an outer annular zone of bonded granular abrasive material of required grindingcharacteristics and having substantially the same volume percentages of bond, grain and pores throughout and an inner strengthening zone of bonded granular material of predetermined diiferent characteristics which is integrally united with the outer zone, the bond and granular material of the inner zone being of such compositions and pro-.-

' wholly of the outer zone composition and structure, the ratio of the moduli ofelasticity of the two zones between 1 and 2.6.

2. A grinding wheel of the type covered. by claim 1 in which the bond in each zone is of the same class and the granular materials and the proportions of the ingredients of the two zones are so related that the moduli of elasticity of the two zones are substantially equal.

3. A composite grinding wheel of the type covered by claim 1 in which the bonded materials of the two zones are such that they have substantially equal coemcients of expansion and substantially equal drying and heat maturing shrinkages whereby the two zones remain integrally united during manufacture of the wheel.

4. A composite grinding wheel as claimed in claim 1 in which the bonded materials of the two zones are made of the samekind of abrasive material and bond, and the proportion of bond to abrasive in the inner zone is greater than that in the outer zone.

5. A composite grinding wheel of the type covered by claim-1 in which the bonded materials of the two zones are made of the same kind of the average size of material in the outer zone.

that with respect to the kind and quantity of bond and the nature of the granular material in the inner zone the latter has a greater strength and ruptural deformation than has the outer zone.

6. A grinding wheel of the type covered by claim -1 in which each zone is made of abrasive grains united by a vitrified ceramic material and the drying and firing shrinkages and the coefiicients of expansion of the two zones are so nearly alike, within the elastic limits of the zone materials, that rupture between the two zones is not caused by such shrinkages and difierential contraction during manufacture of the wheel.

7. A grinding wheel of the type covered by claim 1 in which the inner zone extends radially outwardly to that point at which it receives such a proportion of the stress due to rotation or peripheral heat that the wheel is materially strengthened.

8. A composite grinding wheel comprising an outer annular zone of abrasive material and bond of required grinding characteristics and having a substantially uniform structure throughout, and an inner zone formed in situ of material having such a coefilcient of expansion and maturing shrinkage relative to similar characteristics of the outer zone that the two zones are integrally united during manufacture, and the compositions and proportions of the materials of the two zones being such that the ratio of the moduli of elasticity of the two zones lies between 1 and 2.6 and that the inner zone has a higher strength and ruptural deformation than has the outer zone, and the inner zone being of sufficient size radially as to receive such a proportion of the stress due to rotation or peripheral heat that the wheel is materially stronger than one made wholly of the outer zone composition and structure.

area-34s 9. A composite grinding wheel of the t covered by claim 1 in which the bond in each zone isa vitrified ceramic bond and the quantities and types of granular material and bond are so chosen with respect to the sizes of granular material used that the drying and firing 1 .1 ,es and ture which has an arbor hole therethrough and which has an outer zone and a relatively narrow inner annular zone adjacent the arbor hole to which has been added a strengthg material throughout the thickness of the wheel, the outer zone being unchanged.

11. A grinding wheel of uniform basic structure which has an arbor hole throughout and which has an outer zone and a relatively narrow inner annular zone adjacent the arbor hole impregnated throughout the thickness of the wheel with a strengthening material, the outer annular zone being unimpregnated and unchanged.

12. lhe grinding wheel as set forth in claim 11, in which the strengthening material is a thetic resin.

13. The grinding wheel as set forth in claim 11,

in which the strengthening material is'a phenolic condensation product. 

